Aqueous ink is able to reduce the risk of fire or toxicity such as mutagenicity in association with use of oil-based ink, thereby finding wide use for inkjet recordings other than industrial uses.
Such aqueous ink includes dyes as coloring agents, which are high in stability, less frequent in nozzle clogging, excellent in color developing properties and able to provide a high-quality printing. However, there is a problem that dyes are inferior in water resistance and light fastness of an image.
In order to solve this problem, an attempt has been actively made to shift coloring agents from dyes to pigments. Pigment ink is expected to be excellent in water resistance and light fastness but has problems with nozzle clogging associated with coagulation and sedimentation of pigments. Therefore, such a method has been studied that a pigment processed into fine particles is dispersed into an aqueous medium by using a polymer dispersant.
In order for the ink containing pigments as coloring agents to be used to provide a multi-color printing, all four ink colors of black, cyan, magenta and yellow need to satisfy not only color developing properties but also excellent dispersibility, jettability and storage stability. Therefore, a detailed examination has been made for the selection of optimal pigments according to respective colors, the selection of polymer dispersants capable of dispersing these pigments favorably and stably as well as a method for producing aqueous pigment dispersions using these pigments and dispersants. However, in addition to the variety of various types of pigments corresponding to the respective colors, the details of an optimal dispersion method to be used differ according to each pigment, and optimal ink for inkjet recording is not necessarily provided for all the colors.
In recent years, such an attempt has been made that ink having colors of red, orange, green and violet, in addition to four ink colors of black, cyan, magenta and yellow, is used to improve the color reproducibility of a printed image. Further, such pigment ink for inkjet recording has been demanded for a greater number of colors that is able to satisfy the dispersion stability and that is also excellent in color developing properties and gloss.
Three colors of orange, green and violet are in combination with the above-described basic four colors by using a pigment with a favorable color developing area, thereby making it possible to form an image which is remarkably expansive in the color gamut and excellent in color reproducibility.
However, it is not easy to select a pigment which retains an ideal color developing area and also has the above-described inkjet appropriateness and also provide ink for inkjet recording which has excellent dispersibility, jettability and storage stability.
Since, for example, the color of orange is used in R (red) which is one of the three primary colors of light, many pigments are exemplified such as C. I. Pigment Red 166, Pigment Red 168, Pigment Red 177, Pigment Red 224, Pigment Red 254, Pigment Orange 36 and Pigment Orange 43, as pigments for paste production which applied to color filter (refer to Patent Document 1).
These orange pigments are mainly used as printing ink for color filters and in most cases not expected for use as inkjet recordings. Therefore, no consideration has substantially been so far made for realizing excellent dispersibility and jettability when these pigments are made into an aqueous pigment dispersion with low viscosity. However, in recent years, an attempt has been made for forming a color filter by an inkjet recording method. Attempts have been partially made where these pigments are applied to an inkjet recording method for forming color filters in which the above pigments are used.
However, as compared with printing of color filters made continuously by using a special machine at a plant where temperatures are controlled, an ordinary printing manufactured by using a general-use inkjet printer under conditions substantially free of maintenance in response to irregular and occasional printing requests in a wide temperature range needs ink for inkjet recording which is high in dispersion stability and long storage stability.
Since the above orange pigments have been actually used in color filters, they are often described in known documents as pigments similarly used as ink for inkjet recording. However, consideration has only been scarcely made for ink for inkjet recording in which orange pigments are used, unlike the basic four colors used in an ordinary inkjet recording. Therefore, despite the fact that orange color ink for inkjet recording is expected to provide a jettability, dispersibility and storage stability which are similar to those of the basic four colors, the orange color ink for inkjet recording is actually not provided as that having satisfactory ink characteristics.
Further, in recent years, there is a growing demand for prolonged stability of color development of a printed image. Still further, in association with increased chances that printed materials for industry are used outdoors, there is also a demand for pigment ink for inkjet recording which is excellent in light fastness as well. Where the ink is expected to be used as ink for thermal jet recording, it is considered essentially necessary to provide the storage stability at further elevated temperatures which will result in at greater demand for the ink.
In order to satisfy the above conditions, a detailed examination is required for a polymer dispersant to be combined, an optimal formulation when the dispersant is used, and a production method using the formulation. However, such consideration has hardly been made so far for high-performance ink for inkjet recording using these orange pigments.
In particular, C. I. Pigment Red 168 is hard and difficult in crushing. Since this pigment contains bromine, it is high in specific gravity, easy in sedimentation in solution and quite difficult in retaining dispersion in low-viscosity ink such as ink for inkjet recording. Further, the pigment easily causes coagulation in such a manner that the direction of the pigments is uniform due to the needle shape of pigment particles. Therefore, in view of obtaining a favorable dispersion, it is very important to adsorb a polymer dispersant on the surface of the pigment and stabilize the dispersion with steric hindrances. Further, although it is necessary to make small the particle size of a pigment for the purpose of realizing a high gloss, a pigment having a needle shape such as the Pigment Red 168 is substantially restricted in decreasing the particle size in a longitudinal direction, and the gloss may be greatly decreased where a favorable dispersion is not attained.
Among orange pigments, the Pigment Red 168 in particular is estimated to have the above-described difficulties in preparation of ink for inkjet, and additional consideration is expected to be required for imparting a favorable dispersibility. Thus, the pigment is hardly considered for use of ink for inkjet recording.
A production example has been disclosed in which the Pigment Red 168 is used to obtain an ink composition for inkjet recording which is less likely to cause coagulation by restricting the concentration of phosphorus in the pigment (refer to Patent Document 2). However, an object of this document is to prevent the coagulation, which is a common problem found in many pigments, an emphasis is given only to the concentration of phosphorous in the pigment, and no consideration is made for the selection of an optimal dispersant used in the Pigment Red 168. Further, no description is made for other important characteristics such as dispersibility, storage stability, and light fastness which are required for a pigment used in ink for inkjet recording.
Various considerations have been separately made for polymer dispersants which are to be combined with pigments. However, there are provided no specific guidelines for selecting dispersants as particularly appropriate resin compositions with respect to pigments developing an orange color including the Pigment Red 168 among many polymer dispersants. For example, one of the documents referring to a polymer dispersant used in ink for inkjet recording has disclosed a method for producing an aqueous pigment dispersion used as ink for inkjet having a step where a styrene/acrylic resin of 50 to 90% by mass of a styrene-based monomer unit and an acrylic acid monomer unit or a methacrylic acid monomer unit with an acid value of 50 to 300, a pigment, a wetting agent and a basic compound are kneaded to prepare a colored and kneaded substance (refer to Patent Document 4). However, the above document does not specify a pigment which can preferably be combined in particular or does not disclose at all about which pigment selected from many known orange pigments can yield orange color ink for inkjet recording having excellent characteristics. In the first place, there are reported only a small number of cases in which orange pigments such as the Pigment Red 168 are used to actually prepare an aqueous pigment dispersion for inkjet recording. In particular, the Pigment Red 168 as special ink for forming a color filter has been described only in several documents (Patent Documents 5 to 8) and in Patent Document 3 in addition to Patent Document 2, among documents covering a method for preparing color filters. No consideration has been made for optimal formulations of aqueous pigment dispersions such as selection of polymer dispersants in these documents. Therefore, if these documents are referenced, it is impossible to prepare by using the C. I. Pigment Red 168 excellent in light fastness and color developing properties, an aqueous pigment dispersion excellent in dispersibility and jettability or favorable in long storage stability when used in a general-use inkjet printer.
Patent Document 1: Japanese Published Unexamined Patent Application No. Hei-10-115709
Patent Document 2: Japanese Published Unexamined Patent Application No. 2001-158864 (Embodiment 3)
Patent Document 3: Japanese Published Unexamined Patent Application No. 2001-108819
Patent Document 4: Japanese Published Unexamined Patent Application No. 2005-048014
Patent Document 5: Japanese Published Unexamined Patent Application No. Hei-08-271715
Patent Document 6: Japanese Published Unexamined Patent Application No. Hei-08-005831
Patent Document 7: Japanese Published Unexamined Patent Application No. 2004-090596
Patent Document 8: Japanese Published Unexamined Patent Application No. 2004-302086